Compressor electrical and control fault diagnostics

Why electrical and control faults are easy to misread

A screw compressor is an electromechanical system. The motor, starter or drive, contactors, protection relays, transducers, solenoids and controller all act on a mechanical load that changes every time the machine loads, unloads or modulates. When something goes wrong, the controller reports what it saw, not why it happened.

That creates three traps that cost plants time and money:

• Electrical symptoms that are really mechanical. A tight airend bearing, a blocked oil cooler or a sticking minimum-pressure valve raises motor current and trips the overload. The trip is electrical; the cause is not.
• Mechanical symptoms that are really electrical. A drifting pressure transducer makes the machine load late or unload early. Operators report “the compressor cannot hold pressure” when the airend is healthy.
• Configuration that looks like a failure. A changed pressure band, a restart delay altered after a service visit or a remote-start input left enabled can produce behaviour that looks like a defect.

The practical rule is simple: do not replace the controller because the display shows a fault. Prove where the fault started.

How the electrical and control chain works

It helps to think of the machine as a chain of links, each of which can be tested on its own.

A fault can begin at any link and show up at another. The job of diagnosis is to find the first link that misbehaved.

Safety comes before any measurement

Electrical diagnosis on a compressor combines electrical stored energy with pneumatic stored energy. Both have to be controlled before anyone opens a panel or loosens a fitting.

• Isolate and lock off every supply, including separate control supplies, panel heaters and any remote-start signal from a sequencer or BMS.
• On machines with a VFD, wait for the DC bus to discharge for the time the drive maker states, then confirm with a rated meter. A dark display does not mean the bus is safe.
• Vent the oil sump, separator vessel and pipework to atmosphere and confirm zero pressure. Close and tag the discharge isolation valve.
• Test before touch, using an instrument with the correct measurement category for the panel.
• Live measurements, where genuinely needed, are done only by a competent person under the site’s permit rules.
• Re-energise under control, with the panel closed and everyone clear, and record what was done.

A step-by-step diagnostic sequence

This is the order our engineers follow on site. Skipping ahead to the controller is the most common reason a fault returns a week later.

1. Capture the event. Photograph the active alarm, then export or photograph the alarm and event history with timestamps. Note running hours, load hours and whether the machine was loading, unloaded or starting when it tripped.
2. Ask what changed. Recent service, a parameter change, electrical work on the plant board, a new large load, a DG changeover or a power cut often explains the fault.
3. Check the supply. Measure line voltages and phase balance at the compressor isolator, ideally under load. Look for a missing phase, a voltage dip at start and neutral or earth problems.
4. Inspect connections. With the panel isolated, check terminal tightness, discolouration, burnt lugs and contactor contact wear. Thermal imaging under load, where available, shows hot joints quickly.
5. Test the motor. Insulation resistance phase-to-earth, winding resistance balance, thermistor circuit continuity and a check of bearing noise and coupling condition.
6. Prove the sensors. Compare the controller’s pressure reading with a calibrated test gauge at the same point, and temperature readings with a reference probe. Check sensor wiring, screens and connectors.
7. Prove the outputs. Confirm that load and unload solenoids receive their signal and actually move the inlet valve, and that blowdown works.
8. Review settings and logic. Compare current parameters against the last known-good record. Check the pressure band, restart delay, star-delta changeover time and any remote or sequencer settings.
9. Only then judge the controller. If supply, wiring, sensors and outputs are proven healthy and the fault remains, the controller or its firmware becomes the likely cause.

Common faults and what usually causes them

Note the high discharge temperature row. Normal discharge temperature on an oil-injected screw is typically about 75–95 °C. A trip above that range is usually a cooling or lubrication problem, and the electrical work is limited to proving the sensor is telling the truth. Our breakdown diagnostics page covers the mechanical side of that triage.

Power quality and the Indian site

Many compressor faults in Telangana, Andhra Pradesh and across India start in the supply rather than the machine. Industrial estates often see voltage dips when neighbouring loads start, unbalance between phases on long feeders, and frequent changeovers between grid and DG set.

• Voltage unbalance raises motor current on the high phase and heats the windings even when average voltage looks normal.
• DG-set operation can produce voltage and frequency dips at star-delta changeover if the generator is lightly sized for the compressor’s starting duty.
• Summer heat of 40–45 °C, and above 45 °C on open sites, raises panel temperature, shortens contactor and electronics life, and pushes motors towards thermal limits. Panel ventilation filters clogged with dust make it worse.
• Dust and moisture in panels cause tracking, corroded terminals and intermittent sensor faults, particularly in cement, stone-crushing and construction sites.

Because Reddy Compressors and Reddy Generators are under the same management, we look at the compressor and the generator together when a fault appears only on DG power.

Controllers, firmware and settings

Modern compressor controllers store more evidence than most plants use. Alarm and event logs, running and load hours, start counts and sometimes trend data show the sequence that led to a trip. Read them before anything is reset.

Treat settings as engineering records:

• Record or back up all parameters before work starts, and again after the job.
• Change pressure bands, timers and protection settings only with the maker’s documentation for that model, and record who changed what and why.
• Never raise a protection setting to stop a nuisance trip. Find out why it trips.
• When a controller must be replaced, the replacement must match the model, firmware and configuration of the machine, and sensors must be checked against the new unit before the machine returns to service.

Where machines are linked to a sequencer, master controller or BMS, check who has command authority. A machine that starts on its own may simply be obeying a remote signal. See our machine controller guide for how local control, protection and interfaces fit together.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand compressor specialist based at Tarnaka, Secunderabad, serving industry since 2002. We are not an authorised dealer of any brand, and we diagnose and repair electrical and control faults on machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and many others. We have tie-ups with all major and reputed brands.

• On-site fault diagnosis following the supply-to-response sequence above, with a written record of measurements and findings.
• Starter, contactor, overload, sensor, solenoid and wiring repairs, and motor checks including insulation resistance.
• Controller settings review and replacement controllers, sensors and electrical parts identified from the machine’s nameplate and fitted only once compatibility is confirmed.
• Breakdown cover and rental compressors from our own fleet while a fault is being repaired.
• Service and AMC so that panel cleaning, terminal checks and settings records become routine.

Our service teams cover the whole of Telangana and Andhra Pradesh, with more than 20 service engineers and technicians. Parts and rental machines are supplied across India.

What to send us

• Compressor make, model and serial number, with a nameplate photo.
• Photo of the active alarm and, if possible, the alarm or event history.
• Starter type (star-delta, soft starter or VFD) and motor rating.
• Whether the machine runs on grid, DG set or both.
• When the fault started and what changed recently: service, settings, electrical work.
• Your site location and how critical the machine is to production.

Frequently asked questions

Why does my screw compressor keep tripping on motor overload?

Overload trips are usually a symptom. Check supply voltage and phase balance at the compressor first, then system pressure, oil temperature and cooler condition. A fouled cooler, high working pressure or a tightening airend or motor bearing all raise motor current. Only after those are ruled out should the overload relay or its setting be suspected. Never raise the setting to stop the trip.

How do I know if the compressor controller is faulty?

A controller is proven faulty only after the supply, control transformer, wiring, sensors and outputs have been checked and found healthy. Compare its pressure and temperature readings with calibrated reference instruments, confirm that output signals reach the solenoids, and review settings against a known-good record. If the fault persists with everything around it proven, the controller or its firmware becomes the likely cause.

Why does my compressor trip only when running on a generator?

Generators respond to the large starting current of a compressor motor with a dip in voltage and frequency, especially at star-delta changeover. If the DG set is small relative to the motor, or its governor and voltage regulator respond slowly, the compressor sees phase-failure, undervoltage or overload trips. Measuring voltage during start and reviewing generator sizing usually explains it.

Can a faulty pressure sensor stop a compressor from loading?

Yes. If the transducer reads higher than the real pressure, the controller believes the system is full and keeps the machine unloaded, or loads it only briefly. Damaged cables, water in the connector or a blocked sensing line cause similar behaviour. Compare the controller reading with a calibrated test gauge at the same point before replacing anything else.

Is it safe to reset a compressor alarm and restart?

Only once you know what caused the alarm. Photograph the alarm and its history first, because resetting can clear the evidence. Repeated restarts after high temperature, overload or abnormal noise can turn a repairable fault into airend or motor damage. If the cause is unclear, isolate the machine and arrange a diagnosis.

Does hot summer weather cause compressor electrical faults?

It contributes to them. Ambient temperatures of 40–45 °C, common in Telangana and Andhra Pradesh summers, raise panel and motor temperatures, shorten the life of contactors and electronics, and bring motors closer to thermistor limits. Clogged panel filters and poor compressor-room ventilation make this much worse, so ventilation and panel cleaning matter.

Related pages

• VFD diagnostics and drive fault analysis
• Controller, sensor and instrument reliability
• Breakdown diagnostics and fault triage
• Sensors, transducers and solenoids
• Compressor controllers and HMI parts
• Compressor annual maintenance contracts